Civil Engineering ETDs

Publication Date

9-3-2013

Abstract

Discharge and sediment supply are the primary controls affecting the ability of rivers to adjust. When these are modified to manage water resources for human needs, the ecosystem that relied on them is often negatively impacted. The river that once knew a specific dynamic equilibrium is forced to change. The result is usually changes in hydraulic geometry, a decrease in heterogeneity, bed armoring, disconnected floodplains, vegetative establishment and ecological impacts. Environmental flows are a way to find a middle ground for water resource managers and the physical and ecological needs of the river. The Rio Chama, between El Vado Dam and Abiquiu Reservoir was designated as a Wild and Scenic River in 1988. Along with the desire to protect this reach, came the hope to manage and protect the habitat of brown trout that were introduced for sports fishing. The Rio Chama Instream Flow Assessment, completed by the Bureau of Land Management, aimed to determine flows necessary for brown trout habitat for all life stages but failed to determine the flows that would be required to maintain appropriate sediment conditions for the brown trout and macroinvertebrate food sources (BLM, 1992). The objectives of this study were to spatially evaluate incipient motion of sediments for environmental flows and to determine the flow required to mobilize the channel bed. To accomplish the objective, topographic data collected at the Archuleta and Cebolla sites was used to develop a 2-D mesh in Surface-water Modeling Solution (SMS) and model hydraulic conditions using the Bureau of Reclamations Sediment and River Hydraulics Two-Dimensional Model (SRH-2D). Wolman Pebble counts were completed at each site and particle sizes ranged from < 2 to 256 mm along the intermediate axis. Discharge for model runs within SRH-2D ranged from 14 m3/s to 170 m3/s in 14 m3/s increments. Results from SRH-2D were used to calculate the particle size of incipient motion using Neill's method (1968) and implicit calculations of Shields equation and the Shields-Rouse dimensionless diameter equations (Guo, 2002). It was determined using Neill's method that for 85 m3/s and 170 m3/s, incipient motion for the full range of particle sizes is possible. For 85 m3/s, the mean particle size of entrainment at Archuleta and Cebolla is 41 mm and 57 mm, respectively. However, there is significant spatial variability within each reach and it is likely that the predominant mode of transport is partial movement for all flows. This research suggested that there is a strong connection between channel geometry and the ability of a channel to transport sediments at a given flow. The modeling efforts for this study showed that the most effective use of environmental flows for sediment transport would resemble a natural flow regime in terms of variability. Variability of flood size would accomplish movement of a broader range of sediment size classes as spatial conditions changed with distance downstream of a dam.'

Keywords

Sediment transport--Rio Chama (Colo. and N.M.), Bed load--Rio Chama (Colo. and N.M.), Brown trout fisheries--Rio Chama (Colo. and N.M.), Fish habitat improvement--Rio Chama (Colo. and N.M.)

Document Type

Thesis

Language

English

Degree Name

Civil Engineering

Level of Degree

Masters

Department Name

Civil Engineering

First Committee Member (Chair)

Coonrod, Julie

Second Committee Member

Meyer, Grant

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